GE UR Series L90, Instruction Manual, Page: 630 / 846

10-26 L90 Line Current Differential System GE Multilin
10.8 INSTANTANEOUS ELEMENTS 10 APPLICATION OF SETTINGS
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10.8INSTANTANEOUS ELEMENTS 10.8.1 INSTANTANEOUS ELEMENT ERROR DURING L90 SYNCHRONIZATION
As explained in the Theory of Operation chapter, two or three L90 relays are synchronized to each other and to system fre-
quency to provide digital differential protection and accurate measurements for other protection and control functions.
When an L90 system is starting up, the relays adjust their frequency aggressively to bring all relays into synchronization
with the system quickly. The tracking frequency can differ from nominal (or system frequency) by a few Hertz, especially
during the first second of synchronization. The 87L function is blocked during synchronization; therefore, the difference
between system frequency and relay sampling frequency does not affect 87L function. However, instantaneous elements
have additional error caused by the sensitivity of Fourier phasor estimation to the difference between signal frequency and
tracking frequency.
To secure instantaneous element operation, it is recommended either to use FlexLogic as shown below to block the instan-
taneous elements during synchronization, or to use a different setting group with more conservative pickup for this brief
interval.
Figure 10–5: FLEXLOGIC TO BLOCK INSTANTANEOUS ELEMENT DURING 87L STARTUP
The elements must be treated selectively. If, for example, the phase undervoltage setting includes margin sufficient to
accommodate the maximum additional error on startup, blocking or delay are not needed for phase undervoltage. Similarly,
if the phase instantaneous overcurrent setting has sufficient margin, blocking is not needed. Note that significant zero-
sequence and negative-sequence current or voltage error will not appear during L90 startup, therefore all elements using
these quantities are safe.
The table below indicates the maximum error and recommended block durations for different elements.
ELEMENT MAXIMUM ERROR ON STARTUP,
(OPERATE SIGNAL VS. SETTING)
RECOMMENDED BLOCK
DURATION
Phase undervoltage 18% 0.7 seconds
Phase instantaneous overcurrent 9% 0.5 seconds
Ground distance zone 1 7% Not needed
Phase distance zone 1 4% Not needed